1. Field of the Invention
The present invention relates to the making of downhole force measurements during the drilling of a well bore. More particularly, this invention relates to an apparatus for sensing the amount of weight and/or torque being applied to the drill bit during the drilling operation.
2. State of the Art
It is well known that the axial load and torque applied to a drill bit during the drilling of a well are important parameters which affect the direction and inclination of the borehole as well as the economics of the drilling operation.
The axial load on the drill bit is also known as the "weight-on-bit" or "WOB". Weight is applied to the drill bit by a string of heavy drill collars that are attached immediately above the drill bit and suspended in the borehole on a smaller diameter drill pipe. In conventional drilling practice, the entire length of the drill pipe and the upper portion of the drill collar are suspended at the surface by a derrick in tension so that the amount of WOB can be adjusted by changing the surface hook load. WOB affects the rate of penetration, the drill bit wear and the direction of drilling. The torque applied to the drill bit ("torque-on-bit" or "TOB") is also important with regard to drill bit wear and drilling direction, particularly when considered together with measurements of WOB. Excessive TOB is indicative of serious bit damage such as bearing failure and locked cones.
In the past, measurements of WOB and TOB were made at the surface by comparing the "hook load weight" to the "off-bottom weight" of the drill string and by measurement of the torque applied to the drill string at the surface. It was soon discovered that surface measurements of WOB and TOB are simply not reliable since other forces acting on the drill string interfere with surface measurement.
More recently, various systems have been devised for taking measurements "down-hole" and transmitting these measurements to the surface during the drilling of the borehole. Nevertheless, the down-hole sensors that have been utilized are also subject to significant inaccuracies due to the effects of well pressure and temperature gradients. These systems cannot distinguish between strain due to weight and axial strain due to pressure differential, the force on the end area of the drill string which urges the drill string to elongate under internal pressure. They are also adversely affected by the pressure exerted by drilling fluids.
Several attempts have been made to improve the accuracy of down-hole measurements. One approach, as taught by U.S. Pat. No. 3,686,942 to Chatard et al., is to mechanically constrain a sensitive transducer so that it cannot be deformed and damaged at high loads. Nevertheless, since the transducer-bearing member must be capable of supporting significant axial and/or torsional loads, the transducer is relatively insensitive to minor load changes which are still important to measure. Another approach, as taught by U.S. Pat. No. 3,968,473 to Patton et al., is to provide an inner mandrel with a thin section on which strain gages are mounted and an outer stabilizing sleeve. The compromise between strength and sensitivity is achieved by mathematically sizing the elements. The results from this approach are less than satisfactory. Still another approach, as taught by U.S. Pat. Nos. 3,827,294 to Anderson, 3,876,972 to Garrett, 4,608,861 to Wachtler et al., and 4,811,597 to Hebel, is to provide a mechanical strain amplifier. In these patents, sensors are carefully arranged on stress members which coact with other stress members to produce a sensor response at relatively minor load changes while using a sensor capable of resisting heavy loads. The sensor arrangement is sometimes mechanically isolated by various sleeves and tubes in attempts to eliminate interference from internal bore pressure, temperature and fluids. However, the mechanical amplifier approach is complex and does not adequately isolate the sensors from the effects of thermal expansion.
Both U.S. Pat. Nos. 4,359,898 to Tanguy et al. and 4,821,563 to Maron teach an arrangement of strain gages in a pair of diametrically opposed radial holes in the wall of a drill collar sub carried in the drill string above the bit. Ovalization of the radial holes due to axial or torsional forces on the drill collar or housing is measured by the strain gages on the inner surface of the radial holes. The strain gages are located in positions to minimize interference from temperature gradients and drilling fluid. Tanguy et al. requires a pressurized axial sleeve extending above and below the radial holes and a sealed tube extending across the bore of the drill string which supports end plugs and, in conjunction with the end plugs, establishes an atmospheric chamber in which the strain gages are located in order to eliminate spurious stresses in the region of the holes caused by pressure differences inside and outside the housing. Maron, on the other hand, dispenses with these complicated sleeves and chambers by asymmetrical positioning of the strain gages to minimize errors in the weight measurement caused by pressure changes in the drilling fluid. Maron places six strain gages 45 degrees apart from each other on the inner surface of one radial hole leaving a one hundred thirty-five degree gap between the first and sixth strain gage. In the other hole, Maron places four strain gages 45 degrees apart from each other and a fifth and sixth strain gage respectively 90 degrees apart from the first and fourth strain gage. The strain gages are wired together in three bridge circuits (four strain gages for each), one for measuring TOB, one for measuring WOB, and one for measuring bend on bit.
Both Maron and Tanguy et al. achieve significant improvements in the accuracy of down-hole measurements. However, the arrangement of strain gages on the inner surface of the radial holes is difficult to assemble and maintain. Moreover, the strain gages are subject to measurement drift because of the residual stress in the cold worked drill collar material on which they are mounted.